more works of art.
At each of these sites we find traces of one or two kinds of modern behavior, but never of the whole suite of activities that becomes familiar after 50,000 BCE . Nor is there much sign yet that the modern-looking activities were cumulative, building up gradually until they took over. But archaeologists are already beginning to feel their way toward an explanation for the apparent baby steps toward fully modern humanity, driven largely by climate change.
Geologists realized back in the 1830s that the miles-long, curving lines of rubble found in parts of Europe and North America must have been created by ice sheets pushing debris before them (not, as had previously been thought, by the biblical flood). The concept of an “iceage” was born, although another fifty years passed before scientists understood exactly why ice ages happen.
Earth’s orbit around the sun is not perfectly round, because the gravity of other planets also pulls on us. Over the course of a hundred thousand years our orbit goes from being almost circular (as it is now) to being much more elliptical, then back again. Earth’s tilt on its axis also shifts, on a 22,000-year rhythm, as does the way the planet wobbles around this axis, this time on a 41,000-year scale. Scientists call these Milankovich cycles, after a Serbian mathematician who worked them out, longhand, while interned during World War I (this was a very gentlemanly internment, leaving Milankovich free to spend all day in the library of the Hungarian Academy of Sciences). The patterns combine and recombine in bewilderingly complex ways, but on a roughly hundred-thousand-year schedule they take us from receiving slightly more solar radiation than the average, distributed slightly unevenly across the year, to receiving slightly less sunlight, distributed slightly more evenly.
None of this would matter much except for the way Milankovich cycles interact with two geological trends. First, over the last 50 million years continental drift has pushed most land north of the equator, and having one hemisphere mostly land and the other mostly water amplifies the effects of seasonal variations in solar radiation. Second, volcanic activity has declined across the same period. There is (for the time being) less carbon dioxide in our atmosphere than there was in the age of the dinosaurs, and because of this the planet has—over the very long run and until very recently—steadily cooled.
Through most of Earth’s history the winters were cold enough that it snowed at the poles and this snow froze, but normally the sun melted this ice every summer. By 14 million years ago, however, declining volcanic activity had cooled Earth so much that at the South Pole, where there is a large landmass, the summer sun no longer melted the ice. At the North Pole, where there is no landmass, ice melts more easily, but by 2.75 million years ago temperatures had dropped enough for ice to survive year-round there, too. This had huge consequences, because now whenever Milankovich cycles gave Earth less solar radiation, distributed more evenly across the year, the North Pole ice cap would expand onto northern Europe, Asia, and America, locking up more water, making the earth drier and the sea level lower, reflecting back more solar radiation, and reducing temperatures further still. Earth then spiraled downinto an ice age—until the planet wobbled, tilted, and rotated its way back to a warmer place, and the ice retreated.
Depending on how you count, there have been between forty and fifty ice ages, and the two that spanned the period from 190,000 through 90,000 BCE —crucial millennia in human evolution—were particularly harsh. Lake Malawi, for instance, contained just one-twentieth as much water in 135,000 BCE as it does today. The tougher environment must have changed the rules for staying alive, which may explain why mutations favoring braininess began flourishing. It may also explain why we have
At each of these sites we find traces of one or two kinds of modern behavior, but never of the whole suite of activities that becomes familiar after 50,000 BCE . Nor is there much sign yet that the modern-looking activities were cumulative, building up gradually until they took over. But archaeologists are already beginning to feel their way toward an explanation for the apparent baby steps toward fully modern humanity, driven largely by climate change.
Geologists realized back in the 1830s that the miles-long, curving lines of rubble found in parts of Europe and North America must have been created by ice sheets pushing debris before them (not, as had previously been thought, by the biblical flood). The concept of an “iceage” was born, although another fifty years passed before scientists understood exactly why ice ages happen.
Earth’s orbit around the sun is not perfectly round, because the gravity of other planets also pulls on us. Over the course of a hundred thousand years our orbit goes from being almost circular (as it is now) to being much more elliptical, then back again. Earth’s tilt on its axis also shifts, on a 22,000-year rhythm, as does the way the planet wobbles around this axis, this time on a 41,000-year scale. Scientists call these Milankovich cycles, after a Serbian mathematician who worked them out, longhand, while interned during World War I (this was a very gentlemanly internment, leaving Milankovich free to spend all day in the library of the Hungarian Academy of Sciences). The patterns combine and recombine in bewilderingly complex ways, but on a roughly hundred-thousand-year schedule they take us from receiving slightly more solar radiation than the average, distributed slightly unevenly across the year, to receiving slightly less sunlight, distributed slightly more evenly.
None of this would matter much except for the way Milankovich cycles interact with two geological trends. First, over the last 50 million years continental drift has pushed most land north of the equator, and having one hemisphere mostly land and the other mostly water amplifies the effects of seasonal variations in solar radiation. Second, volcanic activity has declined across the same period. There is (for the time being) less carbon dioxide in our atmosphere than there was in the age of the dinosaurs, and because of this the planet has—over the very long run and until very recently—steadily cooled.
Through most of Earth’s history the winters were cold enough that it snowed at the poles and this snow froze, but normally the sun melted this ice every summer. By 14 million years ago, however, declining volcanic activity had cooled Earth so much that at the South Pole, where there is a large landmass, the summer sun no longer melted the ice. At the North Pole, where there is no landmass, ice melts more easily, but by 2.75 million years ago temperatures had dropped enough for ice to survive year-round there, too. This had huge consequences, because now whenever Milankovich cycles gave Earth less solar radiation, distributed more evenly across the year, the North Pole ice cap would expand onto northern Europe, Asia, and America, locking up more water, making the earth drier and the sea level lower, reflecting back more solar radiation, and reducing temperatures further still. Earth then spiraled downinto an ice age—until the planet wobbled, tilted, and rotated its way back to a warmer place, and the ice retreated.
Depending on how you count, there have been between forty and fifty ice ages, and the two that spanned the period from 190,000 through 90,000 BCE —crucial millennia in human evolution—were particularly harsh. Lake Malawi, for instance, contained just one-twentieth as much water in 135,000 BCE as it does today. The tougher environment must have changed the rules for staying alive, which may explain why mutations favoring braininess began flourishing. It may also explain why we have
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